(1) Field of the Invention
The present invention relates to a thermosensitive reversible recording material capable of color-developing and color-erasing colored images under controlled heating conditions and of holding the color-developed condition and the color-erased condition of the images at room temperature. More particularly, the present invention relates to a thermosensitive reversible recording material capable of forming colored images having a high contrast on a white ground, having a good image-retention characteristic and capable of being repeatedly color-developed and color-erased many times, without the contrast being decreased.
(2) Description of the Related Art
Generally, since a thermosensitive recording apparatus is compact and inexpensive and is easy to maintain, a thermosensitive recording material has been used as an output sheet for computers, measurement equipment, registers, CDS and ATMS, facsimiles, automatic ticket vending machines, and hand-held terminals and, recently, as a magnetic and thermosensitive recording card for prepaid cards and point cards, in which a magnetic recording facility is added. In conventional magnetic, thermosensitive recording cards, while the magnetic information can be renewed at every use, the thermally recorded images cannot be renewed and additional information, for example, remaining number of uses, must be additionally recorded in a portion of the card free from the recorded images. However, since the area of the portion of the card capable of receiving the additional information is limited, the amount of the thermally recorded information is limited or, when the additional information-recording area is filled, the card must be replaced by a new one. Accordingly, to solve the above-mentioned disadvantages, the development of a thermosensitive reversible recording material capable of repeatedly renewing the record of information has been strongly demanded.
Also, based on a recent emphasis on waste-treatment and forest-preservation, re-use of the thermosensitive recording material is desired. There have been various attempts at developing a re-usable thermosensitive recording material. Especially, development of a thermosensitive reversible recording material capable of repeatedly renewing the recorded information many times has been demanded for wide use as a means which does not need a large-scale apparatus, for example, an ink-removing apparatus.
Also, the thermosensitive reversible recording materials as disclosed in Japanese Unexamined Patent Publications No. 3-233,490 and No. 5-42,762 have attracted public attention as recording materials for simple display, and a thermosensitive reversible recording material suitable for the above-mentioned display apparatus is strongly demanded.
On the basis of the above-mentioned demands, various types of reversible thermosensitive recording materials have been proposed.
For example, in Japanese Unexamined Patent Publications No. 63-107,584, No. 4-78,573 and No. 4-358,878, reversible thermosensitive recording materials utilizing a polymer capable of changing transparency thereof in response to heating conditions are described. However, since these recording materials utilize a transparent-to-opaque changing phenomenon due to a phase transition of the polymer, a satisfactory transparency and a sufficient opacity cannot be easily obtained, the contrast between the color-developed images and the color-erased image traces is low, and naked-eye observation in the dark is difficult. Also, in general, the above-mentioned type of recording materials are disadvantageous in that since white images are recorded on a colored ground of the recording material, a recording material capable of recording colored images on a white ground, namely, a paper-like recording material, is difficult to obtain.
As means for solving the above-mentioned problems of the conventional reversible thermosensitive recording materials, a dye-type reversible thermosensitive recording material using a conventional dye usable for conventional thermosensitive recording materials and a dye developer capable of reversibly color-developing and erasing the dye are known. The dye-type reversible thermosensitive materials can easily record colored images in a white ground, and the resultant recorded images have relatively high contrast. As the above-mentioned dye type reversible thermosensitive recording material, the following systems are known.
In Japanese Unexamined Patent Publications No. 58-191,190 and No. 60-193,691, a system using, as a developing agent, gallic acid or phloroglucinol is described. However, this type of system is disadvantageous in that since erasure of the colored images needs water or water vapor, the color-erasing apparatus must have a large size.
Japanese Unexamined Patent Publications No. 60-264,285 and No. 62-140,881 disclose a system using a thermochronic material with hysteresis. This type of system is disadvantageous in that since the colored image-retaining temperature range is limited in both the upper and lower limits thereof, the apparatus for the system is complicated and there is a limitation on the temperature range usable for practice.
Japanese Unexamined Patent Publication No. 63-173,684 discloses a system using, as a color-developing agent, an ascorbic acid derivative. This system is disadvanteous in that, in erasing the colored images, erasure cannot be fully effected.
Japanese Unexamined Patent Publications No. 2-188,293 and No. 2-188,294 disclose a system in which a salt of a specific organic acid such as gallic acid with a higher aliphatic amine is used as a color-developing agent. This system is, however, disadvantageous in that since the color-developing reaction and the color-erasing reactions are competitive with each other, it is difficult to control the reactions so as to selectively promote only one of the reactions, and colored images with a high contrast are difficult to obtain.
Japanese Unexamined Patent Publications No. 5-124,360 and No. 6-210,954 disclose a system using, as a color-developing agent, a phosphoric acid compound or phenol compound each having a long chain alkyl group. This system is, however, disadvantageous in that the colored images may be not fully erased and the storage property of the colored images may be insufficient.
As mentioned above, although various types of reversible thermosensitive recording material have been disclosed, each of them has various disadvantages. Therefore, no reversible thermosensitive recording material having a practically satisfactory performance has been obtained.
To solve the above-mentioned problems, the inventors of the present invention have provided a thermosensitive reversible recording material in which a specific color-developing agent comprising a compound having a long chain alkyl group and a sulfonyl(thio)urea group is used, as disclosed in Japanese Unexamined Patent Publication No. 9-272,261.
This type of thermosensitive reversible recording material can form colored images having a high contrast and an excellent storage retention on a white ground by using a small, compact printer. The colored images can be repeatedly color-developed and color-erased only by changing the heating condition. However, the recording material is not satisfactory in that an increased number of repeats of the color-developing and erasing operations may cause the erasure of the colored images to become insufficient and thus after the color-erasure, the color density of the resultant residual images to increase.